Nancy Bartow, a federal government biologist, wearing hip-high fisherman's boots, climbed out of her outboard motorboat into the murky Potomac River south of Woodrow Wilson Bridge, searching for a small underwater bed of wild celery.

At low tide near the Maryland shore, the river was only about two feet deep. Bartow bent down and pulled up a few slender green leaves.

"They're healthy. They're starting new plants," she said. Then she waded around, trying to gauge the bed's size. She concluded that it was about five feet square. A second bed nearby appeared to be just two feet square. "It the celery really likes it here for some reason," Bartow added.

The wild celery offered a tantalizing clue to the health of the Potomac River. Decades ago, such underwater plants grew profusely near the shore in the Washington region. By the mid-1950s these aquatic grasses virtually had disappeared, apparently ravaged by pollution, storms or other causes. Today, a key issue for the Potomac is whether the plants ever will return.

Despite massive scientific research, much remains uncertain about the lower stretches of the Potomac, including Washington's waterfront. More than $1 billion has been spent to revitalize the waterway since 1965, when president Lyndon B. Johnson launched an enormous cleanup drive, terming the river a "disgraceful" example of "decaying sewage and rotten algae."

The prevailing view among scientists and government officials is that the river has shown considerable improvement, especially in such key water-quality indicators as dissolved oxygen levels. But some critics dispute those claims. In an atmosphere of fiscal austerity, many government officials pointedly ask how clean must the river be?

A new study by the Metropolitan Washington Council of Governments (COG) has raised doubts about whether the cleanup goals that were established more than a decade ago ever will be achieved. County and city officials, seeking to reduce multimillion-dollar costs, are calling for easing the standards governing discharges into the river from sewage treatment plants.

Johnson's pledge to "reopen the Potomac for swimming by 1975" remains unfulfilled. Nor do officials believe that the next target date, 1983, will bring full compliance. Next July 1 marks the Clean Water Act's deadline for making the river clean enough for fishing and swimming. But District of Columbia officials, concerned about safety and health hazards, are not expected to lift their ban on swimming by that date.

To some extent, the changing outlook on the river reflects the uncertainties of science.

"Our state of understanding of the art is, I don't want to say rudimentary -- it's developing. It's at the forefront of knowledge," said James P. Bennett, a U.S. Geological Survey hydrologist who is coordinating a five-year, $6 million study of the lower reaches of the Potomac from the Washington area to the Chesapeake Bay.

While fishermen tout the river's "rebirth," researchers admit that they have virtually no scientific data to indicate a significant increase in fish, shellfish or other wildlife. The little bed of wild celery below Wilson Bridge, planted by Geological Survey biologists as an experiment, may indicate a possible upturn, but scientists express uncertainty.

Smelly, unsightly mats of algae have not appeared on the river in recent years, and floating bits of untreated sewage are less evident. Officials point out, however, that raw sewage still overflows into the river from sewer pipes more than 45 times a year, usually after rainstorms. Washington-area governments have vastly reduced the amount of sand, silt and clay that once poured into the river from constuction sites and other sources, but the river continues to carry a sizable load of such suspended sediments, partly as a result of land erosion upstream. The amount of dissolved oxygen in the Potomac has increased, but officials say the level still occasionally drops below the limits considered essential to fish and other aquatic life. Coliform counts, an indicator of possible disease-causing organisms, have fallen, yet microbiologists say that bacteria frequently are too numerous in the river to permit swimming. Chlorophyll measurements, an indicator of the amount of nuisance algae in the water, have decreased, but remain far in excess of the long-established goal.

"You hear people say the river is all cleaned up. And you hear people say it isn't cleaned up. And, in effect, they're both right," said Fairfax County Supervisor Audrey Moore, who has listened to much of the technical debate.

The two small underwater plots of wild celery growing near Rosier Bluff point up the uncertainties. Researchers have tried for several years to establish wild celery beds at various spots near the shore, using special cages to protect the young plants from predators and other harm. Several clusters survived during the summer of 1981. But when the protective cages were removed for the winter, most of the plants perished.

"These are the only two beds that came back on their own this year," Bartow noted, as she waded about on a gray autumn afternoon near two metal stakes jutting from the water to mark the beds' locations.

By midsummer, moreover, virtually all of the leaves in the two surviving beds had been nipped off, perhaps by carp, other fish or muskrats. Scientists have no clue as to what animal or other cause is responsible for the clipped leaves, although the plants seem rugged enough to withstand the snipping. "Keeps getting bitten off and comes right back," Bartow remarked.

One other bit of evidence is a large bed of wild celery that has grown naturally, with no help from scientists, along the Washington Channel, a waterway separated from the Potomac by East Potomac Park and joining it at Hains Point in Southwest Washington. This bed was noted by Geological Survey researchers in 1981, but it is not known for how many years these plants have survived or whether they are becoming stronger and more profuse.

Should the wild celery beds be interpreted as favorable signs for the Potomac? "It's a tough damn question," replied Virginia Carter, a veteran Geological Survey biologist overseeing the submerged vegetation studies. The transplanted grass near Wilson Bridge may signify an improved outlook "if it goes on growing," Carter said.

Scientists do not know, for example, whether similar transplants might have taken root five or 10 years ago, when the Potomac appeared more polluted. Nor are they certain of the extent to which pollution affects the underwater grasses, although it is considered likely that some pollutants, such as phosphorus, may stimulate development of tiny organisms that block the light needed for plants to grow.

The submerged grasses are an indicator of ecological balance and a link in the river's life. They provide food and shelter for ducks, fish, shellfish and other animals. Their roots help to stabilize the river bottom; their leaves retard the current. They provide oxygen to the waterway through photosynthesis.

Another vital indicator is the amount of oxygen dissolved in the water, where it allows fish and other aquatic animals to survive.

Dissolved oxygen in the Potomac has been the subject of a special study because of the startup of a new sewage-treatment process, known as nitrification, at the District of Columbia's large Blue Plains plant, whose discharges significantly affect the river's health. Nitrification is a biochemical process that combines oxygen with ammonia through bacterial action to produce other nitrogen-containing substances, called nitrites and nitrates. If this reaction were not carried out at sewage plants, it would occur naturally in the Potomac and deplete the oxygen in the river.

"The really startling improvement in the Potomac occurred in 1981, because they started nitrification at Blue Plains," said Paul W. Eastman, executive director of the Interstate Commission on the Potomac River Basin. Two recent research findings, one by COG and the other by the Geological Survey, appear to support this viewpoint.

Counts of fecal coliform, a form of bacteria found in sewage that indicate the possible presence of disease-causing organisms, have dropped markedly in recent decades, officials said. But the counts fluctuate widely and frequently exceed health standards, especially when raw sewage overflows into the Potomac because of storm-overloaded sewers.

Rita R. Colwell, a University of Maryland microbiologist who is studying the Potomac, said she would be "reluctant to swim in the Potomac . . . within the District of Columbia." Her tentative findings indicate high numbers of bacteria, including organisms that may cause dysentery or similar illnesses. "I would just exercize caution."

Scientists say they have little substantive evidence to show that the recent popularity of fishing along the river has been matched by an increase in the fish population. Fish data are not compiled for the Washington stretch of the river, officials noted, and marine statistics for the lower portions of the Potomac tend to reflect a decrease that has occurred throughout the Chesapeake Bay region in such key fish species as striped bass.

Perhaps the only scientific finding hinting at any upturn in Potomac fish is a report by Environmental Protection Agency researchers that points to an increase in juvenile white and yellow perch. The agency termed this "a notable exception" to downward trends elsewhere in the bay area.

Gordon Leisch, a sport fisherman, biologist and Interior Department administrator, has conducted his own survey of fishing in the Washington area. While unscientific, it may be as reliable an indicator as any that exists, and his results appear mixed.

Since 1975, Leisch has counted his own catch during 200 to 300 days a year of fishing. His largemouth bass tally dropped from an average of about 200 fish in earlier years to about 100 since 1979. Meanwhile, his haul of striped bass, or rockfish, climbed from fewer than 60 a year in the mid-1970s to more than 200 in 1980 and 1981. At the same time, his catch of white perch has held fairly steady at about 200 a year.

"There should be some qualifiers thrown in here," Leisch noted. Part of the upturn in Leisch's rockfish and the downturn in largemouth bass, he said, merely reflects Leisch's recent efforts to net more rockfish.

Yet, on the whole, Leisch saw some meaning in the crisscrossing statistics. "The largemouth bass fishing has always been good here," he said. "I think for rockfish, there is a trend there . . . . We're getting more rockfish and bigger rockfish."